Current and future telecommunication switches make extensive use of backplane connectors. These connectors are used to join various system components of a communication switch together. Due to the strong desire on the part of the marketplace for high pin density, these connectors are frequently constructed such that signal pairs are placed in close proximity to one another. A problem that results from this arrangement is that the electric and magnetic fields of signals traversing through the connector interact with one another, causing an undesirable crosstalk between the signals. As data rates continue to increase, the problem of crosstalk will also increase.
High speed serial backplanes are a common media used in the creation of network switches. The backplane is a very thick printed wire board (PWB) or printed circuit board (PCB) with many internal layers whose surface is populated with connectors. The backplane is typically a passive board that serves to interconnect the various connectors on the surface of the board. In general the arrangement of the pins is such that there are rows of pins in a ground-signal-signal-ground (GSSG) arrangement. Adjacent columns of these GSSG will interact both electrically and magnetically. Line cards comprising additional printed circuit boards having dedicated integrated circuits mounted on them are coupled to the backplane. The capabilities of the IC's coupled with the connectivity of the backplane form a complete switch that is capable routing digital data.
Crosstalk can occur in several ways. For high speed communication, two wires are used per data line. The wires themselves can support two forms of EM fields, an even mode and an odd mode. For high-speed digital data transmission, differential signaling is used. Differential signaling corresponds to the odd mode. The receiver in a high-speed differential data transmission system is designed to detect odd mode signals. Along the pair, changes in field configuration occur when there is a physical transition. This can occur for example when the striplines in the backplane hit the backplane connector vias, or signals traveling in the vias hit the actual backplane connector. One impact of these discontinuities is mode conversion (i.e. when an even mode signal is converted into an odd mode signal). A consequence of mode conversion is that it is possible for even mode noise to be converted into an odd mode signal that will be detected at the receiver.
For a victim aggressor scenario where both the victim and aggressor are differential pairs, there are two primary ways for crosstalk to manifest itself on the receiver. Differential signals on the aggressor can affect the odd mode of the victim. Also, even mode signals from the aggressor couple to the victim and the victim pair converts the even mode signal into an odd mode signal. Reducing the ability of modes to couple would substantially reduce the crosstalk.
Network switches enable the transfer of serial digital data within this system at data rates in excess of 1 Gbps (gigabit per second). Many popular forms of encoding of the serial data result in signal frequencies in excess of 1 GHz. At such high frequencies, crosstalk of signals becomes a barrier to effective error free communication. A significant source of crosstalk occurs within the vias of the backplane and printed circuit boards.
Accordingly, there is a need for an improved printed circuit board and method of reducing crosstalk in a printed circuit board.